helper/command: remove unused field in struct command_registration

The field jim_handler_data in struct command_registration is never
assigned, thus has always value NULL. It was added in commit
17a9dea53a71 ("add jim_handler to command_registration") on Nov 23
2009, together with the homonym field jim_handler_data in struct
command, but never used since then.
Only the field jim_handler_data in struct command is used.

Remove the field from struct command_registration and use NULL
where it was referenced (or remove the assignment if the recipient
is already zero, e.g. allocated with calloc()).

Removing the field decreases the total size of OpenOCD binary by
only 4944 byte on a 64 bit x86. Not a significant improvement from
this point of view.

Change-Id: I9f1d281e3de6b2eb398e2d883c5e9ff92628aecd
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5225
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>